This invention relates to a method of cooling steel pipes that improves the cooling capacity of the cooling bed by the forced cooling of tempered pipes and/or enhances the collapse strength of pipes without increasing their tensile strength by said forced cooling.
Many quenching methods have been proposed for the heat treatment of steel pipes. Most of these methods comprise cooling steel pipes, first heated to above the Ac.sub.3 transformation temperature (e.g. 850.degree. C.), down to below approximately 100.degree. C., by passing the pipes through one or more ring headers carrying many nozzles to spray cooling water. To reduce the addition of costly hardenability enhancing elements, these quenching methods call for such techniques as can assure extremely high cooling capacities (such as not lower than 35.degree.-40.degree. C./sec. in terms of the mean cooling rate at the inside pipe wall surface). For this purpose, various techniques have been proposed that involve such conditions as the average water flux of not less than 3 m.sup.3 /min.m.sup.2 and the mean heat transfer coefficient of not less than 8000 kcal/m.sup.2.h..degree.C. Here, the average water flux means the quantity of water supplied to the cooling zone divided by the external surface area of the pipe being cooled in the cooling zone. That is, the average water flux q (m.sup.3 /min.m.sup.2) can be expressed as q=Q/.pi.DL, in which D (m) is the outside diameter of the pipe, L (m) is the length of the cooling zone (or the part of the pipe which is exposed to the water), and Q (m.sup.3 /min) is the volume of water supplied. In other words, q represents the amount of cooling water sprayed per unit time onto unit area of the external surface of the pipe.
After being thus rapidly quenched, the pipe is reheated to between 500.degree. and 700.degree. C., at which temperature the pipe is held for a short time to provide what is known as tempering. Then, usually, the pipe is allowed to cool on the cooling bed down to the vicinity of 100.degree. C. or room temperature very slowly, under the condition analogous to natural convection cooling. But this cooling of the tempered pipes on the cooling bed takes a long time, so that improvement of the cooling bed's capacity or development of a new, more efficient cooling method has come to be needed in order to cope with the recent increase in demand for high-quality heat-treated pipes.
Also, if the coolapse strength of pipe can be increased by giving properly selected cooling after tempering, such a method is favorable to the manufacture of oil-country tubular products and the like upon which great pressure is likely to be exerted.
Various methods of cooling heated pipes have been invented, but they have all been intended for other purposes than quenching. For example, Japanese Patent Publications Nos. 94415 of 1979 and 34667 of 1970 disclose methods to cool heated pipes from inside. Their object is to make the size of pipe steel crystal grains finer or enhance the corrosive resistance of pipe by relieving the internal pressure through the application of compressive residual stress on the inside of the pipe. So these methods cannot meet the aforesaid requirements. Japanese Patent Publication No. 80211 of 1979 discloses a method of cooling pipe from outside. This invention, like the present one, relates to a forced external cooling method, but is principally intended for crystal grain refinement like the foregoing two internal cooling methods.